Among other things, the setting above has the effect of making
unary-- ``invisible'' for the purposes of applying permutative
:rewrite rules to binary-+ trees. Thus, arg and (unary-- arg) will
be given the same weight and will be permuted so as to be adjacent.
The form (invisible-fns-table (w state)) returns the current value
of the invisible functions table.

where alist is either t or a true list of pairs, each element of
which is of the form (fn ufn1 ... ufnk), where fn is a function
symbol and each ufni is a unary function symbol. When alist is t,
the initial value of this table is used in its place. Modulo the
replacement of alist by the default setting when alist is t, this
macro is equivalent to